Support materials for an imaging element often employ layers comprising glassy, hydrophobic polymers such as acrylics, styrenics, and cellulose esters, for example. One typical application is as a backing layer to provide resistance to abrasion, scratch, blocking, and ferrotyping. For coating applications, the glassy polymers are normally dissolved in a solvent at very low solids to ensure low coating solution viscosities for good coatability at high coating speeds on a moving film support. Coating techniques employed include single or multilayer extrusion dies (commonly referred to as X-hoppers), air knife, roller coating, meyer rods, knife over roll, and so on.
For coating solutions comprising soluble polymers of reasonably high molecular weights, for example, larger than 50,000, the solution viscosity is a strong function of polymer concentration. For example, Elvacite 2041, a methyl methacrylate polymer sold by E.I. DuPont de Nemours and Co., has been described in the photographic art to form scratch protective layers for photographic materials. The polymer is normally dissolved in an organic solvent such as methylene chloride to form a clear solution. At concentrations above, for example, 4 to 5 weight %, the Elvacite 2041 solution viscosity is at least 20 centipoise at ambient temperature. Those viscosity values are too high for coating applications by, for example, certain roller coating or air-knife coating techniques, which require a coating solution viscosity in the range of from one to several centipoise. Therefore, photographic manufactures have to keep the solid concentration low to provide low solution viscosities and good coatability at high coating speeds.
Polymer solutions with low solids are useful for applications where lower dry coating coverages (less than about 300 mg/m.sup.2) can meet the physical and mechanical properties requirements for an imaging system. However, more advanced imaging applications need higher dry coating coverages for better physical and mechanical properties. To obtain high dry coating coverages, either more coating solution per unit area (wet coverage) has to be applied when using low viscosity/low solids polymer solutions, or higher viscosity/higher solids solutions must be used. As stated above, however, many coating applications cannot tolerate high viscosity/ high solids polymer solutions, as such solutions cannot be coated at low wet coverages at high coating speeds. Some coating methods may allow one to coat high viscosity polymer solutions at high wet coverages, but they still suffer from several disadvantages. For example, in general, higher wet coverages mean more solvent recovery and higher cost for drying. Furthermore, due to both manufacturing limitations and various physical and mechanical property requirements for imaging element, wet coverages cannot be increased under certain conditions and for certain applications. For example, high wet coating coverages and the high levels of solvent retained in the film support as a result of these high wet coverages may have a significant impact on both dimensional stability and sensitometric properties of an imaging element. One may use resins of low molecular weight to lower the solution viscosity. However, the resultant dry coatings may not have adequate physical and mechanical properties.
Alternative approaches employing low viscosity, dispersed polymer particle-containing coating compositions have been described for paint and automotive coating industries. For example, U.S. Pat. No. 4,336,177 describes a solvent coating composition comprising non-aqueous dispersible composite polymer particles larger than 0.1 .mu.m. The particle has a core with a glass transition temperature (Tg) of about 10.degree. C. less than the polymerization reaction temperature. The particles are stabilized by block or grafting copolymers and can be transferred directly from aqueous medium to a non-aqueous medium. U.S. Pat. No. 4,829,127 describes a coating composition comprising composite resin particles. Such particles are prepared by solution polymerization techniques in reaction vessels containing initiator, solvent, polymerizable monomers, and crosslinked particles. U.S. Pat. No. 3,929,693 describes a coating composition comprising a solution polymer and polymer particles, where the polymer particles have a crosslinked rubbery core below 60.degree. C. and a grafted shell having molecular weight of 1,000 to 150,000. Reportedly, such coating compositions are more stable toward premature separation and flocculation. U.S. Pat. No. 3,880,796 describes a coating composition comprising thermosetting polymer particles containing insoluble microgel particles having a particle size of from 1 to 10 .mu.m. U.S. Pat. No. 4,147,688 describes a dispersion polymerization process of making crosslinked acrylic polymer microparticles having a particle size of from 0.1 to 10 .mu.m. U.S. Pat. No. 4,025,474 describes a coating composition comprising a hydroxy-functional, oil-modified or oil-free polyester resin, aminoplast resin, and 2 to 50% of crosslinked polymer microparticles (0.1 to 10 .mu.m) made by dispersion polymerization process. U.S. Pat. No. 4,115,472 describes a polyurethane coating composition comprising an ungelled hydroxy-containing urethane reaction product and insoluble crosslinked acrylic polymer microparticles (0.1 to 10 .mu.m) made by a dispersion polymerization process. Such coatings are reportedly useful for automotive industries.
There are significant differences in designing coating compositions for photographic applications from those for paint and automotive coating industries. The coating techniques and coating delivery systems are different so that they need different coating rheologies. The drying time in exterior and interior paint and architectural coating applications is on the order of hours and days, and in the automobile industry on the order of 10 to 30 min. However, in the photographic support manufacturing process the drying time for coatings is typically on the order of seconds. Often the drying time for solvent-borne coatings is as brief as 10-30 seconds for high speed coating applications. These differences put additional stringencies on the coating composition for photographic materials. For example, the coating viscosity frequently needs to be on the order of less than about 10 centipoise, and more often less that 5 centipoise, instead of on the order of one hundred to several thousand centipoise as in other coating industries. A typical dry coating thickness for photographic materials is on the order of less than 2 .mu.m, and more often less than 1 .mu.m. Film formation and dried film quality are especially critical. The tolerance on defects caused by polymer gel slugs, gelled particles, dust, and dirt is extremely low. This requires special precautions in delivery processes. The coating solutions need to be very stable toward, for example, high speed filtration and high shear.
U.S. Pat. Nos. 5,597,680, 5,597,681, and 5,695,919 describe coating compositions for imaging elements that contain core-shell polymer particles dispersed in liquid organic medium. Such coating compositions are stable and have low viscosity at high solids. However, there is a need to provide organic solvent based coating compositions that yield dried layers with even superior physical and mechanical properties compared with these core-shell polymers.
Aqueous coating compositions comprising water dispersible polymer particles have been reported to be useful for some applications For example, they have been used as "priming" or subbing layers on film support to act as adhesion promotion layers for photographic emulsion layers, and used as barrier layers over, for example, a vanadium pentoxide antistatic subbing layer to prevent the loss of antistatic properties after film processing as described in U.S. Pat. No. 5,006,451. U.S. Pat. No. 5,679,505 describes an improved motion picture print film with a protective overcoat containing a polyurethane. Preferably the polyurethane is a water dispersible polyurethane. While these coating compositions are attractive from environmental considerations, the slow evaporation rate of water coupled with its extremely high heat of vaporization causes drying problems which are either not normally encountered or can be easily overcome in solvent-borne systems. Therefore, for manufacturing processes with conventional organic solvent drying capacity, the use of water-borne coating compositions often leads to very unsatisfactory results. In addition, solvent based coatings are preferred when the substrate or layer to be overcoated are moisture sensitive.
It can be seen that various approaches have been attempted to obtain useful organic solvent-based coating compositions with low viscosity and high percent solids. While the aforementioned prior art references relate to some aspects of the present invention, they are deficient with regard to simultaneously satisfying all the physical, chemical, and manufacturing requirements for a solvent-borne coating for more advanced imaging applications. The present invention provides coating compositions, and imaging elements containing a layer coated from such coating compositions, which meet all of these requirements while avoiding the problems and limitations of the prior art.